Interest in hydrofluoroalkanes has grown since chlorofluoroalkanes (CFCs) have been suspected as contributing to the weakening of the stratospheric ozone layer.
CFCs, such as fluorotrichloromethane (CFC 11), dichlorodifluoromethane (CFC 12), 1,1,2-trichloro-1,2,2-trifluoroethane (CFC 113) and chloropentafluoroethane (CFC 115), have for this reason been banned in ail industrialized countries and have been replaced by hydrofluoroalkanes, such as 1,1,1,2-tetrafloroethane (F134a), difluoromethane (F32), pentafluoroethane (F125), 1,1,1,2,3,3,3-heptafluoropropane (F227ea), 1,1,1,3,3-pentafluoropropane (F245fa) and 1,1,1,2,2-pentafluorobutane (F365mfc), and by hydrochlorofluoroalkanes (HCFC), such as chlorodifluoromethane (F22), 1,1-dichloro-1-fluoroethane (F141b) and 1-chloro-1,1-difluoroethane (F142b). Although less harmful than CFCs to the ozone layer, HCFCs ire, however, destined to gradually disappear. It is therefore necessary to be able to manufacture products of HFC type in order to replace CFCs and HCFCs.
The majority of processes currently known for the synthesis of HFCs are based on the catalytic fluorination of chlorinated compounds using hydrogen fluoride or on the hydrogenolysis of a chlorofluorinated compound or on the pyrolysis of an HCFC in the presence of HFCs. It is obvious that all these processes, which involve a chlorinated product and which coproduce hydrochloric acid, give, as impurities, chlorinated products of HCFC or CFC type which are not very desirable because of their effects on the ozone layer. The content of chlorinated Impurities of CFC or HCFC type in the HFCs thus manufactured must be as low as possible and is thus an important factor to be taken into consideration.
Due to the various applications of these products (refrigeration, air conditioning, expansion of foams, solvent or extinguishing of fires), it may prove necessary to have available several HFCs possessing different physical and chemical properties or mixtures of HFCs more specifically satisfactory for certain applications.
It would be particularly advantageous in this field to have available a process making possible the manufacture of several HFCs with different physical properties from the same starting material without resulting in the formation of chlorinated byproducts and impurities of HCFC or CFC type or of hydrochloric acid, in particular for manufacturing pentafluoroethane (F125) and 1,1,1,2,3,3,3-heptafluoropropane (F227ea).
Mention may be made, among the numerous processes for the synthesis of F125 from a chlorinated or chlorofluorinated derivative, of:
those relating to the fluorination of 1,1,1-trifluoro-2,2-dichloroethane (F123) by hydrogen fluoride in the gas phase in the presence of a catalyst containing chromium deposited on a charcoal support (Patent EP 456,552) or in the presence of a catalyst containing chromium on a support of fluorinated alumina type (Patent EP 349,298), PA1 the disproportionation of an HCFC derivative, such as F124, the passage of which over a catalyst of chromium oxide type results, at the outlet, in a mixture of F125 and F123 (Patent Application WO 9202476), PA1 the hydrogenolysis of F115 (Application WO 9105752 and Patent EP 0,506,525), a degree of conversion of F115 of greater than 99% requiring forcing conditions which result in the formation of significant amounts of F143a (CF.sub.3 --CH.sub.3). PA1 (a) a stage which consists in subjecting, in the gas phase, a stream of trifluoromethane (F23) to pyrolysis at a temperature of greater than 700.degree. C., and PA1 (b) a stage which consists in bringing the mixture of gases which result from the pyrolysis stage into contact with a fluorination catalyst.
These processes, involving chlorinated derivatives of the HCFC or CFC type, generally involve a thorough purification of the F125 obtained, one of the major problems being the presence in the F125 of F115, which is difficult to separate by simple distillation and which has to be removed by more complex techniques, as described in Patent ER 2,716,449.
F125 can also be obtained by fluorination of tetrafluoroethyline in the liquid phase (Patent U.S. Pat. No. 4,258,225) or gaseous phase (Patent EP 0,036,123) in the presence of catalysts, but these processes require the isolation or the storage of C.sub.2 F.sub.4, which constitutes a major handicap because of the dangers of explosion or off polymerization inherent in this product. According to Patent RU 2,049,085, the fluorination of C.sub.2 F.sub.4 as a mixture with F124 can be carried out in the gas phase with a catalyst based on chromium on an aluminium oxide: C.sub.2 F.sub.4 and F124 can result from the pyrolysis of F22 (CHClF.sub.2) but the F125 thus obtained is contaminated by HCFCs, such as F114 and F115, which greatly reduces the advantage of such a process.
Patent ER 2,731,701 describes a process for the synthesis of F125 from a mixture of F23 and F22. Although this process makes it possible to produce other HFCs than F125, it requires the use of a chlorinated starting material, F22; moreover, the presence of HCl formed by the reaction is also not very desirable, because of the problems of corrosion and of the chlorinated byproducts which it can generate in the reaction mixture.
Patent U.S. Pat. No. 3,009,966 describes a process for the manufacture of perfluorinated olefins comprising a stage of pyrolysis of F23 and indicates that very small amounts of F125 and F227ea are also formed.
Mention may be made, among the various processes for the synthesis of F227ea, of the hydrogenolysis of 2-chloroheptafluoropropane over metal catalysts (Patent EP 539,989) and the fluorination by HF of perfluoropropene in the presence of an antimony-comprising catalyst (Patent Application WO 9602483).